Duplicate loading observables

[cmckni3]

Currently, the datatable contains a loading observable that is set before loading data from the server. There is also a showLoading computed which just returns the value of the loading observable.

We should eliminate the showLoading computed and just have the loading observable.

For now, we could just point showLoading to the loading observable for backwards compatibility and note that showLoading is deprecated.

[cmbankester]

Assigned it to the 1.0.0 milestone since this is technically a non-backward-compatible feature